I currently am running Linux Mint x64 edition, but would like to switch to the Main edition. If I have a dual-boot with Windows XP set up, and a manual partitioning set up, how would I install the Main edition over what I currently have?

Just select the partition where you currently have the old version of Mint installed. Do the "edit" partition and identify it as "/". Make sure you check the format check box. Make other changes that you want as far as partitions mounted at bootup while in the manual partition section of the install.